This is quite unethical, and I'm amazed it was published at all. Kashmir Hill at Forbes nails it:

While many users may already expect and be willing to have their behavior studied — and while that may be warranted with “research” being one of the 9,045 words in the data use policy — they don’t expect that Facebook will actively manipulate their environment in order to see how they react. That’s a new level of experimentation, turning Facebook from a fishbowl into a petri dish, and it’s why people are flipping out about this.

Shocking stuff. We need a new social publishing platform, built on ethical, open systems.